Imaging arrays of the charge transfer type, such as the charge coupled device disclosed by Wallace in U.S. patent application Ser. No. 297,055 filed Aug. 27, 1981, now U.S. Pat. No. 4,362,575, issued Dec. 7, 1982, which is incorporated herein by reference, store light-generated electrical charge and transfer such charge to a charge detector for detection and display. The array generally includes a semiconductor body of a first conductivity type having a plurality of substantially parallel charge transfer channels of opposite conductivity type extending a distance into the body from a first major surface thereof. The charge transfer channels are isolated from one another by channel stops which provide potential barriers to the flow of stored charge between adjacent channels. Each channel stop is generally composed of a region which extends a distance into the body from the first major surface between a pair of charge transfer channels which has the same conductivity type as the body but has a higher concentration of conductivity modifiers. A dielectric insulator overlies the first major surface with a plurality of substantially parallel electrodes overlying the dielectric layer and extending in a direction transverse to the direction of the charge transfer channels.
Application of a voltage of the proper polarity to a particular electrode will cause light-generated electrical charge to accummulate in a potential well in the charge transfer channel under the electrode. The sequential application of voltages to successive electrodes over a channel will cause the charge to be transferred to the charge detector.
If the amount of charge generated in a particular portion of the array is in excess of the amount which can be stored in the potential well formed under a particular electrode, the excess charge will spread into adjacent potential wells along the charge transfer channel. This spreading of the charge causes an increase in the size of the brightest portions of an image and is known as blooming. A solution to the blooming problem is to reduce the concentration of conductivity modifiers in the channel stops and to form blooming drains in the channel stops which have the opposite conductivity type to that of the semiconductor body. The lighter doping of the channel stops reduces the height of the potential barrier between the potential wells in the channels and the blooming drains so that the excess charge preferentially flows over the barrier and into the blooming drain rather than over the higher barriers along the charge transfer channel. The drains significantly reduce the blooming but at the price of also reducing the light sensitivity of the array since a portion of the charge generated in the semiconductor body, rather than flowing to the potential wells in the channels, flows directly into the drains and is lost. Thus it would be desirable to have an imaging array having blooming control but without the corresponding loss of sensitivity.